1. Field of the Invention
The present invention relates to a ladder filter, and more particularly to a ladder filter having a plurality of edge-reflection type surface acoustic wave resonators.
2. Description of Related Art
A surface acoustic wave filter (SAW) in which a plurality of SAW resonator are arranged to define a ladder circuit is known and referred to as a ladder filter. FIG. 1A shows a conventional ladder filter 201 disclosed in Japanese Laid-open patent application No. 5-183380, and FIG. 1B is an equivalent circuit thereof. The ladder filter 201 includes a piezoelectric substrate 202 and series one-port SAW resonators 203 and 204 and parallel one-port SAW resonators 205 to 207 disposed on the piezoelectric substrate 202. The series one-port SAW resonators 203 and 204 are connected in series between an input terminal IN and an output terminal OUT to define a series arm, and parallel one-port SAW resonators 205 to 207 are respectively connected in parallel between the series arm and a ground potential to each define a parallel arm.
As shown in FIG. 1A, the SAW resonators 203 to 207 each includes a respective pair of interdigital transducers (IDTs) 203a to 207a and a respective pair of grating reflectors 203b to 207b disposed on opposite sides thereof. In the SAW resonators 203 to 207, surface acoustic waves excited by the IDTs 203a to 207a are confined between the grating reflectors 203b to 207b so as to form standing waves. Each of the resonators 203 to 207 has a resonance characteristic in which the impedance of the resonator is low in the vicinity of a resonant frequency and the impedance is high in the vicinity of an antiresonant frequency.
In the ladder filter 201, the resonant frequency of series one-port resonators 203 and 204 are constructed to be coincident with the antiresonant frequency of the parallel one-port resonators 205 to 207. Thus, the ladder filter 201 is provided within a passband defined by the antiresonant frequency of the series one-port resonators 203 and 204 and the resonant frequency of the parallel one-port resonators 205 to 207.
The conventional ladder filter 201, and the ladder filter of this type has been widely applied to a TV set, VCR, communication device such as a cellular phone or the like. However, there has been a continuous demand for improving the performance of such a ladder filter and miniaturizing such a ladder filter.